It has been known to use complexes composed of titanium atoms and organic ligands as catalysts in asymmetric reactions. It has been also known that optically active compounds, in particular, axially asymmetric compounds containing hydroxyl groups, are selected as the organic ligands therefor.
However, many of these axially asymmetric compounds containing hydroxyl groups are very expensive and, therefore, unsuitable for industrial uses. Accordingly, it is highly advantageous from the industrial viewpoint, if possible, to add a catalytic amount of an optically active axially asymmetric compound (i.e., a chiral activator) to a complex with the use of a ligand in racemic modification to thereby exclusively activate one of the enantiomers of the complex, thus achieving catalytic asymmetric synthesis with the use of the catalyst thus obtained.
There have been reported several cases of the asymmetric synthesis with the use of asymmetric ligands in racemic modification or metal complexes as precursors. For example, J. M. Brown et al. (J. Chem. Soc., Chem. Commun., 1986 p. 1532) reported that CHIRAPHOS (2,3-bisdiphenylphosphinobutane) in racemic modification was treated with an optically pure (S)-iridium complex to thereby form its complex with (R)-CHIRAPHOS while the residual (S)-CHIRAPHOS was employed as a rhodium complex in the asymmetric hydrogenation of dehydroamino acids. According to H. Yamamoto et al. (J. Am. Chem. Soc., 1989, vol. 111, p. 789), a chiral ketone was added to a racemic binaphthol/aluminum complex to thereby inactivate the (R)-binaphthol/aluminum complex via the formation of a complex, while the residual (S)-aluminum complex was employed in the asymmetric Diels-Alder reaction. Furthermore, J. W. Faller et al. (J. Am. Chem. Soc., 1993, vol. 115, p. 804) reported that a phosphine ligand originating in methionine was added as an inactivating agent to a racemic rhodium complex to thereby inactivate one of the enantiomers, thus effecting the asymmetric hydrogenation of dehydroamino acids. However, it seems that the inactivated enantiomers are wasted in these three cases. In addition, these methods are not efficient, since any enantio-selectivity exceeding the level achieved by using optically pure catalysts cannot be established thereby.